Conventionally, there has been known a gas sensor which detects the concentration of a particular gas contained in a mixed gas. However, in some cases, the characteristic of a gas sensor which represents the relation between the concentration of the particular gas contained in the gas and a concentration signal output from the gas sensor in accordance with the concentration of the particular gas (hereafter, the characteristic will be referred to as the particular-gas concentration characteristic) may vary slightly among gas sensors due to a variation involved in production of gas sensors.
According to a known technique, in view of the above, for each of a plurality of gas sensors, one of a plurality of previously set ranks is assigned to the gas sensor on the basis of its particular-gas concentration characteristic, and a label resistor having a resistance corresponding to the assigned rank is attached to the gas sensor (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2005-315757). This technique makes it possible to determine the particular-gas concentration characteristic of a gas sensor to be used on the basis of the resistance of the label resistor and accurately calculate the concentration of the particular gas using the determined particular-gas concentration characteristic. Notably, the resistance of the label resistor is calculated on the basis of a voltage between opposite ends of the label resistor (hereinafter referred to as the “label resistor voltage”) measured when a resistance measurement current having a current value set in advance is supplied to the label resistor.